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I have an unfinished Basement that smells damp so I put in a humidity meter last summer and it was reading around 80% which is known to cause mold. I'm not seeing the mold but I put in this dehumidifier and it runs 24/7 during the summer and then shuts off on its own when its reached the proper humidity (which only happens maybe late fall/early winter). If I had to guess the basement is about 1000 sq feet. The dehumidifier has a pipe running from it into the drain in the floor so I never have to empty it manually. (humidity meter in the basement today said 70°F and 79% humidity. And the outside temperature today was 72°F). Whenever it rains, the humidity meter is almost guaranteed to be 79%.

The dehumidifier is positioned in a corner/alcove of the unfinished basement near the unfinished space where a bathroom could be built. Is it possible that it's not properly reaching the whole room being in that alcove? if I put it in the center of the room I'll have to run the hose across half the room and not sure gravity will push the water across the room through that hose and into the drain pipe.

A furnace lives a few feet away from the dehumidifier in the basement, but furnace is incapable of heating or cooling the basement. The furnace only heats/cools the first floor of the house.

What do I need to do to get the humidity level to stay well below 80% when the dehumidifier doesn't seem to be doing much during the summer? I noticed last year that if I turn the AC on for the first floor and then open the basement door to let the ac's air get into there, the humidity levels werent as bad. There is no dirt, ice or frost on the filter of the dehumidifier. Should I ask my HVAC guys to run ductwork into the basement in order to give it air flow? Would that help?

House was built in 2002.

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  • Not an HVAC expert, but my thought would be to move the dehumidifier to the center of the basement and see if the results are better. If so, then address the needs to make that location permanent.
    – RMDman
    Commented May 6 at 0:25
  • If your dehumidifier is running 24-7, then it is undersized for the task at hand and you more dehumidifiers or a bigger one, so it always cycles on/off except on the wettest few days of the year. And when you get tired of paying the electrical bill, bring in experts to figure out where the beans all this water is coming from. Commented May 6 at 19:19
  • Also confirm that the dehumidifier is actually pulling moisture out of the air by making sure that water is coming out of the drain line in a good stream or at least fast drip. If the unit is low on refrigerant, it might not be getting cold enough to condense water out of the air on its cooling coils.
    – Milwrdfan
    Commented Jun 7 at 13:56

2 Answers 2

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  • Lower the water table around the house (exterior drains, interior sump, both, or more of each.)

  • Address any issues with the ground not sloping away from the house.

  • Add gutters or make sure gutter downspouts take water well away from the house.

  • Seal the concrete that's acting like a damp sponge to re-humidify the space.

Or get a much bigger dehumidifier.

The latter costs more to operate.

Also cover the sump pit, if any, which is a puddle of water in the basement helping to re-humidify it. You said drain, but some folks are imprecise about the difference in terminology regarding a floor drain .vs. sump pit, so unclear from here if you have both, or which one if only one.

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Airflow! I have been conducting my own experiments, and having enjoyed a huge finished basement in my childhood home, I eventually identified why our big basement, after 30 years, was always flooded and dank. It's groundwater and stormwater flooding downhill during and after downpours (though even a slight decline would also flood foundations and basements). Assuming your basement and house are situated even slightly downhill on your property that faces a paved (impervious) road, one solution would be to dig a trench parallel but angled toward the side of your house where it would continue downhill. The trench needs to be deep and wide enough (at least 18"-24" deep x 2 feet wide) to handle the countless volume of water and direct it toward a stream (even a dry creek bed). You'd install iron gate or heavy gauge flow-through covers on top so no one falls or trips over the trench. Another step (and it's costly, but recommended) is to have impervious heavy gauge plastic or special liner installed on the entire subterranean foundation. This requires digging alongside the foundation that would face the property incline and installing the heavy gauge moisture barrier on the brick or cinder block substructure. This prevents groundwater seepage that get through your foundation and into your basement. Even the slightest groundwater moisture will increase the humidity level in your basement and house. So, a moisture barrier is necessary to prevent water intrusion and also the damage caused by underground moisture. This damage includes destabilizing the foundation causing your house to shift and sink slightly on an end. This would explain cracks you may have noticed on your walls that are not underground. You would negate need of a dehumidifier. Still, if your basement remains dank, install fans. Circulating air is essential to addressing humidity, because moving air also dehumidifies the moisture in the air. Think of your hairdryer. Then, as I have found, determine which side of your house is getting the most sunlight, and which side is shadiest. Is your basement located on the west side? Or even the south side of your house. Reason is, humidity is the result of cold and warm air colliding. So, if your cool, temperate basement has a south or west facing exposed wall with no shade, and you have ventilation, cracks and unsealed windows, doors, or anything through which that warm air is flowing into your nice, cool basement, you will have humidity problems. And if any part of your basement has an open area, such as a crawlspace that runs the span of your house, you will have humidity. The remedy may be to enclose or create a barrier closing that opening to your crawlspace, or, install fans that would exhaust air from your basement into the crawlspace. This would help reduce humidity in your house while you dehumidify your basement using circulation instead of a heat-producing dehumidifier. Which brings me to my final point. Dehumidifiers produce hot/warm exhaust that is released into your cool basement. This creates humidity. (You see how complicated this is?) When you think about how a home's central HVAC works, you'll quickly realize just how many functions they have: Cool the air, circulate air creating airflow directed throughout the house with ventilation and ducts; dehumidify the air (which is accomplished by circulation, and compression and dehumidification which is the primary function of an AC system), and collecting/directing water from condensation in plumbing that is directed far from the house so it does not puddle and seep back in. So, you could have vents and properly sized ducts that enable sufficient force and flow of air into your HVAC. And short of all of this, I would strongly encourage your investing in a small, 9k BTU ductless minisplit, which happens to dehumidify better than any other type of air conditioning. You'll just have to direct the condensate away from your foundation. And whatever you do, use fans. Ceiling fans, box fans, and where applicable, exhaust fans (exhausting hot air out/cool air in). So far, I have been able to make my house quite comfortable strictly based on positioning of fans. I've had to deploy my portable air conditioner a few times, but I have been able to stave off investing in fixing my air conditioning systems. Hope this helps someone.

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    Would you mind also conducting an experiment in the use of paragraphs? It will benefit us all! Commented Jun 7 at 5:43

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